Electrical transmission lines are used to transmit electric energy and signals from one point to another. The basic transmission line connects a source to a load--e.g. a transmitter to an antenna, an antenna to a receiver, or any other application that requires a signal to be passed from one point to another in a controlled manner. Electrical transmission lines, which can be described by their characteristic impedance and their electrical length, are an important electric component in radio frequency (RF) circuits. In particular, transmission lines can be used for impedance matching--i.e., matching the output impedance of one circuit to the input impedance of another circuit. Further, the electrical length of the transmission line, typically expressed as a function of signal wavelength, determines another important characteristic of the transmission line device.
Manipulation of the characteristic impedance and electrical length of the transmission line device is a well known technique to effect a particular electrical result. In particular, an output impedance, Z.sub.out, can be matched to an input impedance, Z.sub.in, according to a well known equation, as later described. Similarly, the attenuation and phase shift of the transmission line device can be altered by changing the physical length of the conductor between the input and output ports of the transmission line device. As an example, a resonant circuit results when the physical length of the conductor approximates an even one-quarter wavelength of the signal's nominal frequency.
Of course, at high frequencies the wavelength is small and transmission line devices can be built using relatively short conductors in small packages. By contrast, as the nominal frequency of the applied signal decreases, the physical length must necessarily increase to effect the desired transmission line characteristic. The physical length must correspondingly increase to accommodate such applications operating at lower frequencies.
Prior art techniques, including microstrip and stripline conductors, have been used successfully in the past to construct transmission line devices. Unfortunately, at lower frequencies--e.g., below 1 GHz--the substrates upon which these one-dimensional conductive strips are placed require a relatively large area, due to the excessive length requirements. As today's electronic devices shrink in size, the board space allotted for the necessary electrical components is correspondingly reduced. Thus, a substrate carrying a microstrip or a stripline conductor that serves as a transmission line device for low frequency signals simply cannot be accommodated by the available board space.
Another technique that is employed can be described as a helical structure disposed inside a grounding cylinder. Such helical coils are well known in the art, but these too are often inadequate for today's applications, where low volume and low cost are critical factors in the manufacture of portable electronic devices. Because of the tight length and impedance specifications, the helical structures become very costly to manufacture. That is, the manufacturing variance that is inherent in the construction of such devices--e.g. conductor diameter, symmetry of windings, and effective number of turns--tends to make the helical structure a less desirable solution for tight tolerance transmission line devices. Further, the cylindrical grounding portion, which feature is required when building a transmission line device, results in a circuit having a relatively large volume, or poor form-factor, that is untenable for many of today's applications.
Of course, as the number of transmission line devices required for a particular circuit increases, so too does the volume required for embodying those devices within the circuit. Aside from the increased volume, though, another undesired effect is promulgated when multiple transmission line devices or even single devices requiring many turns to effect the desired electrical length--are needed in the circuit. Of these problems, one of the most serious tends to be a so called ground-shielding effect. That is, when multiple turns are required for a particular application, the outermost coils-those coils closest to the external ground plane--tend to block, or shield, the innermost conductors from that ground plane. In particular, due to the symmetrical nature of the turns that make up the coil structure, the outermost conductors become an obstacle between the innermost conductors and the ground plane, thereby terminating the electric field lines propagated therebetween.
The obstruction in the electric field path results in a reduced capacitive effect, and therefore an undesirably high inductive reactance, for the structure. This becomes an even greater problem when many transmission line devices, or many turns for each of the devices required, become necessary to effect the desired characteristics for the circuit.
Accordingly, there exists a need for an electrical circuit arrangement that substantially eliminates the problems associated with inter-stage shielding in coil structures for transmission line devices. In particular, such a circuit that provided a less obstructed path between each of the coil sections to ground, for each of the transmission line devices, would be an improvement over the prior art.